Optical illusions

I thoroughly enjoy viewing optical illusions (although many of the ‘moving’ illusions give me strong headaches) and am fascinated about the insight they give into human visual perception, and exactly how our brain interprets the incoming data to present the physical world around us. This particular broad description for these types of illusions actually covers a wide range of individual perceptual interpretations, and a great number of simple illusory tricks.

So, here’s a bit of introductory ‘edutainment’ to start you thinking, tickle your imagination, and open your eyes to the visually confounding and remarkable universe we inhabit.

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A technique fashionable during the Renaissance was the use of distorted perspective, known as anamorphosis, to add optical elements to artwork that demanded the art be viewed ‘unconventionally’ from a specific angle, or with the help of a reflective device. German artist and printmaker Hans Holbein (the Younger) produced one of the most quizzical and widely discussed anamorphic examples with his 1533 painting ‘The Ambassadors’, which now hangs in the National Gallery in London. The foreground of his double portrait contains an optically concealed skull, which to be seen correctly requires the viewer to be very close to the plane of the painting and to the right hand side – only then will the solid three-dimensional form of the skull appear as intended:

'The Ambassadors' skull seen correctly

A large high-res interactive version of the painting is available to view here.

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The relationship of grouped colours and the effects one colour can have on another are of key interest for all designers and artists. Visual context will determine the colour that we perceive, regardless of the actual colour, and fool us to believe we are seeing something different to actuality. These samples below show how neighbouring colours can easily confuse our colour perception system to create a powerful illusion.

Although the left square appears as orange, and the right a deeper brown, both are an identical colour

Both of the lower squares show an identical dark to light blue gradient

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This neat geometrical illusion is a variant of the Ponzo illusion (named after Italian psychologist Mario Ponzo), which shows how our mind perceives an object’s size based on present background information. This illusion highlights how linear perspective within an illustration suggesting convergence, produces the belief that an object in the distance must be larger than that in the foreground.

wall illusion

The two green lines are exactly the same height

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Impossible objects, such as Schuster’s clevis (below) are some of the fantastical ambiguous figures, which have confounded and amused people for generations. The Schuster ‘devil’s fork’ and other grand impossibilities such as the ‘impossible staircase’ (made famous by the M. C. Escher lithograph ‘Klimmen en dalen) are fabulous examples of paradoxical two-dimensional forms bewildering viewers when they attempt to construct them in three dimensions.

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One of my all-time favourite perceptual illusions is Charles Allen Gilbert’s 1892 illustration ‘All is Vanity’ – a classic visual contrivance:

'All is Vanity' by Charles Allen Gilbert

Also, this German postcard from 1888 (artist unknown) requires us to ‘switch’ between perceiving either a young or old woman – visual data that the brain must interpret and construct from the ambiguous information present within the image.

Old or young – which do you see?

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Let’s now take a look at the moderately disturbing anomalous motion illusions. WARNING! – many of these images can cause feelings of sickness and dizziness, due to the effect of rotating movement in what is only a static image. These images ‘move’ due to their combination of very specific element relationships and their relative reflected luminance. A particular intensity of contrast causes our visual system’s motion detecting neurons to react to shape edges, and thereby perceive motion where none exists. Some of these motion illusions create a very uncomfortable viewing experience, but are equally interesting to behold.

Rotating rays

Rotating rays

Primrose's field

Primrose’s field

Staring fixedly at a particular point will slow or stop the movement, but any minor vision shift will start it up again!

The above examples are files created by Professor Akiyoshi Kitaoka, who studies visual perception and illusion at the Ritsumeikan University in Kyoto, Japan. Further brilliant examples are available at his comprehensive website.

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Finally, there are the popular art illusions seen on streets around the world as street paintings and wall murals. There are a great number of fantastic street painters in countries all across the globe, transforming our urban areas from every-day dullness into wildly imaginative visions with mesmerising illustrative technique and some very clever visual ruses.

Siete Punto Uno

John Pugh’s mural ‘Siete Punto Uno’ – Main Street, Los Gatos, California.


Edgar Müller’s ‘Phoenix’ – City Art Gallery, Grand Rapids

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There you have it – my brief tour of the spellbinding world of optical illusions. Many illusions are deliberately set to mess with your mind and have you believing the unbelievable, however, our interpretation of visual data isn’t always the most reliable and trustworthy process, so be on the alert. Seeing isn’t necessarily completely believing!


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